"I should say that historically, there have sometimes been issues with Macs that have had 3rd-party Web Cams attached. If you have any external audio or video attached to your Mac, I'd suggest you unplug them from your Mac, and re-try Mixtikl."

I have finally been able to track-down and fix this bug - having bought a new Web Cam over Christmas purely to try to solve the problem! - and it'll be in the next round of updates.

The problem was if you had a USB device plugged-in, that device only offerred *mono* audio input; where that device was configured by your Mac to be the default audio input device on your machine when it got plugged-in.

Another work around would be to have kept the device plugged-in, but to use Audio MIDI Setup on your Mac, to set the internal Microphone to be your default audio input device (this is stereo, so this would then work fine!).